Coating by cathode disintegration



Feb. 7, 1939.

F. M. PENNING COATING BY CATHODE DISINTEGRATION Filed NOV. 7, 1936 lNvfNTOR FRANS MICH El. PENN l NG ATTORNY UNITED STATES PATENT OFFICE 2,146,025 f COATlvNG BYl CATHODE DISINTEGRATION Frans Michel Penning, Eindhoven, Netherlands,

assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application November 7, 1936, Serial No. 109,618

4 Claims.

` This invention relates to a method of disintegrating a cathode by means of an electric glow discharge in an atmosphere of gas at a low pressure (lower than 0.2 mm.). As nis well known l the pressure of gas and the current-density are in this case such'that the positive ions impinging on the cathode bring about disintegration of the cathode material.

Such acathode disintegration may be effected gfor the purpose of coating bodies with metal films, the body to be coated being arranged so .near the cathode that upon disintegration the icathode particles deposit on it. The body is often arranged on the anode. y

Cathode disintegration may also be used for the purpose of reducing the pressure of a gas in a closed chamber. `The cathode particles disin, tegrated combine with gas molecules and thus bring about a reduction in gas pressure.

According to the invention, the discharge path is subjected to the influence of a magnetic eld and the electrode configuration, the direction and the intensity of rthis magnetic eld are such that the discharge current is substantially higher than in the' absence of the magnetic field, since this magnetic field deflectsthe electrons from the path which they would follow in the absence of the magnetic field and materially increases the total path which the electrons traverse. This results in a greater number of collisions between the electrons and the gas, which would also occur if the gas pressure were increased without magnetic field. The magnetic eid consequently brings about an apparent increase in gas pres- .sure with respect to the characteristic curve of the discharge. As the cathode particles disintegrated are frequently uncharged or if they are charged are but little biased by the magnetic iield by 'reason' of their comparatively large mass, this apparent increase in pressure does not become manifest with respect to the behaviour of the particles disintegrated. It is therefore possible that the gas pressure is quite low and nevertheless acurrent occurs the intensity of which is a multiple of that of the current which would occur at this gas pressure in the absence of the magnetic field. The current intensity becomes A atleast 5 times as high as that which occurs in of disintegration is thus increased. When bodies' are coated with the material disintegrated there `gas pressures, which in Germany December 28, l1935 (Cl. Z50-27.5)

is in addition the particular advantage that it is possible for the material to deposit on the body to be coated in a liner form, that is to say in the form of particles of smaller dimensions.

The electrode configuration and the magnetic 5 field are so chosen that during discharge the electrons are prevented from reaching the anode directly along the electric lines of force so that they traverse a materially longer path than in the absence -of the magnetic field. This may be 10 brought about in various ways.

Reduction of already low gas pressures (for exv ample lower than 50 microns) by means of cathode disintegration has the disadvantage that the the voltage necessary for initiation of the discharge, is' very high. Thus, for example, the starting voltage of a discharge in nitrogen between two large parallel plates spaced 1 cm. apart is, at a pressure of 20 microns, already ap- 20 proximately '70,000 volts. This method may therefore .entail particularly great difficulties in practice.

Thus, according to the invention the electrode configuration and the direction as well as the in- 25 tensity of the magnetic field may be so chosen that not only is the current intensity substantially increased but the starting voltage is also substantially reduced. The invention is therefore also especially suitable for obtaining by 3c cathode disintegration a reduction of already low the absence of a magnetic field would necessitate particularly high starting voltages.

vFor this purpose, the electrode configuration 3g and the direction of the magnetic field may be such that not only during discharge but also during initiationthe magnetic lines of force in at least one part of the discharge space form with the electric lines of force an angle higher than o preferably of 90. The electrons which happen to be in the discharge path and bring about starting up thus traverse under the influence of the magnetic field such a lengthened path that the starting voltage is substantially reduced. 45 The starting voltage may be reduced in a simple manner to a value, for example 1/2 that in the absence of the magnetic field.

Use may be made, for example, of fiat electrodes arranged invv parallel or of electrodes hav ing the same axis and the magnetic eld may be arranged in such manner that the magnetic lines of force are normal to the shortest lines of junction between the electrodes. Such electrode coniigurations permit of ensuring that in the entire l discharge space both during initiation and during discharge the magnetic lines of force are normal to the electric lines of force.

A further possibility consists in the use of a cathode comprising a group of magnetic lines of force not retained by the anode. A simple coniguration is obtained when the cathode is constituted by two plates normal to the magnetic lines of force and the anode is constituted by a wire, a plate or a cylinder parallel to the magnetic lines of force. When a cylindrical anode is used it may entirely surround the space between the -two cathode plates.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by ref erence to the following description taken in connection with the accompanying drawing in which Figure 1 is a longitudinal section of one form of electron discharge device embodying my invention, Figure 2 is a transverse section of Figure 1, Figures 3 and 4 are longitudinal and transverse sections of another form of electron discharge device embodying my invention, and Figures 5 and 6 are longitudinal and transverse sections of a modified electron discharge devicev embodying my invention.

In order that the invention may be clearly understood and readily carried into eifect it will now be described more fully, by way of example with reference to the accompanying drawing in which the figures show a few possible electrode configurations and arrangements ofthe magnet coil that produces the magnetic feld. It is only natural that the electrodes are arranged in a closed discharge vessel. The magnet coils are arranged preferably outside the discharge vessel.

The electrode system shown in Figures 1 and 2 comprises a rod-shaped cold cathode I and an anode 2 by which it is axially surrounded.A The electrodes are surrounded by a magnet coil 3 on the outside of envelope I2 enclosing electrodes I and 2. The electric lines of force extend radially between the cathode and the anode (designated by dotted lines) and the magnetic lines'of force are parallel to the axis of the electrode system (designated by arrows). The magnetic and the electric lines of force are consequently normal to each other throughout the discharge space. Due to the magnetic eld the electrons receive such a deilection from their path that they circle around the cathode so that the path traversediby them is substantially increased. As set out hereinbefore, this results in an apparent increase in gas pressure and a substantial increase in current intensity and in addition vin a reduction of the starting voltage.

In a given form of construction the radius of the cathode cylinder was 0.5 mm. and that of the anode cylinder 15 mms. and the gas filling was constituted by argon at a pressure of 0.1 mm.

The electrodes position of a resistance of direct current of 1000 volts. of a magnetic field the discharge milliampere, whereas with an energized magnet coil constructed in such .manner that the magnetic field intensity inthe discharge pathwas approximately 300 gauss, the discharge current were connected, with the inter-l In the absence was milliamperes. The magnetic field consequently brought about a 300-fold amplification oi' the current..

If in the same device with the same source of current andthe same series resistance the distwo parts interconnected by of 5000 ohms, to a source current was 0.1-

charge current were to be adjusted to 7 milliamperes, this would require in the absence of a magnetic field an argon pressure of 0.133 mm.but in the presence of a magnetic field of about 300 gauss only an argon pressure of 0.06 mm. The magnetic eld consequently brought about under these conditions a from 5 to 6 fold apparent increase in gas pressure.

The cathode I is made of material with which a body to be coated has to be coated. If a lm of tungsten is to be applied to this body, use is made of a tungsten cathode, but of a silver cath- A ode if a film of silver is to be. applied. 'I'he body to be coated is arranged so near the cathode as to be coated with the cathode particles disintegrated. In many cases the body may be arranged on the inner side of the anode. The method may also be employed for the manufacture of particularly thin metal plates. In this case, the cathode particles disintegrated are deposited on some base which after being coated with a metal iilm due to the cathode disintegration is removed from this film.

By reason of the substantially amplified current. a vgreater number of ions impinge on the cathode and the speed at which these ions impinge on it is very high due to the low gas pressure. This leads to strong disintegration of the cathode and the cathode particles disintegrated readily pass through the gas at a reduced pressure.

The electrode system shown in Figures 3 and 4 comprises a rod-shaped cathode l and an anode formed by two round plates 5 and 6 normal to the axis of the cathode enclosed within envelope I3. The magnetic field is set up by a magnetic coil 1 arranged axially of the cathode. When a discharge is being struck it is possible to observe adjacent the cathode 4 the Crookes space and the negative glow. It iswell known that in these parts ofthe discharge the electric lines of force are alwaysnormal to the cathode surface and that in the Crookes space practically the entire potential diilerence between the electrodes is compressed whereas the electric eld in the remaining part of the discharge space is but feeble. 'I'he magnetic lines of forceare parallel to the axis oi' the cathode and adjacent the cathode they are thus normal to the electric lines of force. The electrons are consequently deected and again they circle around the cathode so that the path traversed by the electrons is materially increased. A

Figures 5 and 6 show an electrode system comprising a cylindrical anode 8 and a'cathode formed bytwo round plates, for example iron plates; l and III arranged adjacent the open ends, of the cylindrical anode 8 and enclosed within envelope I4. The cylinder 8 is axially surrounded by a magnet coil II so that the magnetic lines of force in the discharge space are parallel to the axis of the anode.

The electrons emerging from the plate 0 would pass, in the absence of a magnetic eld, along curved paths towards the cylindrical anode. Under the influence of the magnetic eld these electrons describe, however, more or less helical paths around the magnetic lines of force so that these electrons are kept distant from the anode. If the electrons come near the cathode plate II their forward movement is checked and they are repelled towards the plate l. They consequently vpass to and fro between the cathode plates 9 ing between the cathode parts 9 and I0, but may even be smaller. Good results are obtained even with an anode formed by an annular wire.

The important point oi the electrode conguration shown in Figures 5 and 6 is that the two cathode parts are interconnected by a group of lines of force not retained by the anode so that the electrons which tend to follow the magnetic lines of force under the inuence oi the magnetic eld move to and fro between the cathode parts. Under certain conditions these cathode parts may constitute a unitary piece even mechanically.

As set out hereinbefore, in this arrangement the magnetic field also brings about a reduction of the starting voltage. In a given case the diameter 'and the length of the anode were 40 and mms. respectively and the spacing between the plates 9 and l0 whose diameter corresponded approximately to that of the anode was '70 mms. 'I'he current of the magnet coil was so chosen that the magnetic ileld intensity in the anode axis wasl approximately 300 gauss. At a pressure of 10-4 mms. of the gaseous atmosphere of argon the starting voltage was in this case approximately 1000 volts, whereas in the absence of the magnetic eld this voltage was 1500 volts already at an argon pressureof 0.03 mm.

In this arrangement the discharge current intensity is also so chosen that substantially disintegration of the cathode material occurs. The cathode particles disintegrated combine with the gas molecules and thus reduce the gas pressure. The gas pressure may thus be readily reduced down to below 0.01 micron. It is not impossible 'that part of the gas is not taken up by the cathode particles disintegrated but absorbed by the glass walll in the form of ions.

In the arrangements described due regard should not only be paid to the variation of the magnetic lines of force but care should be taken that the magnetic field is sufdciently powerful, since the influence of this ileld is otherwise too low to bring about adequate amplification of the current. `In many cases a permanent magnet may be used for setting up the magnetic field.v

` If a body is to be coated with a mixture of two diierent metals, use may be made of two or more cathodes 4oi' different metals or of a cathode formed by parts of4 diiIerent metals. Thus', for example, the arrangement shown in Figures 5 55 and 6 is particularly vsuitable for this purpose since in this case the two cathode parts 9 and I0 are of different materials.

What I claim is:

1. A method of reducing the time of coating by cathode disintegration in a device having a cathode and an electrode to be coated with a metallic i'llm both within a gaseous atmosphere, comprising establishing a magnetic eld between said cathode and electrode and applying a voltage between said cathode and electrode suiciently great to cause ionization of the gaseous atmosphere, the magnetic eld and electric eld intersecting each other.

2. A method of reducing the time of coating by cathode disintegration in an apparatus having a cathode and an electrode to be coated with a metallic lm, both within a gaseous atmosphere, comprising maintaining the pressure of said' gaseous atmosphere less than 0.2 millimeter,

establishing a magnetic eld between s'aid cathode and electrode, and a voltage between said cathode and electrode sufciently great to cause ionization, the magnetic eld and electric iield intersecting each other.

3. A method of reducing the time of coating a surface with a metallic coating and compris- 'ing positioning a cathode and an electrode whose surface is to be coated in a gaseous atmosphere of low pressure, establishing an electric field between said cathode and electrode and a magnetic eld between said cathode and electrode, said electric field and magnetic ileld being established at angles to each other greater than 45 but not more than whereby the magnetic field and the electric field intersect each other, said voltage being great enough to cause ionization of the gaseous atmosphere. Y

4. The method of reducing the time oi coating a surface with a metallic coating by means of cathode disintegration and comprising positioning a cathode and an electrode within a gaseous atmosphere and parallel to each other, establishing an electric field between said cathode and electrode by applsdng a potential between said cathode and electrode and establishing va magnetic field parallel to"said cathode and electrode and between said cathode and electrodes for increasing the path of travel oi' electrons between said cathode and electrode whereby ionization of the gaseous atmosphere and cathode disintegration can be established on a lower than l -normal voltage, the magnetic field and electric 

